mGlu5 receptor and protein kinase C implicated in the development and induction of neuropathic pain following chronic ethanol consumption

Mechanisms of small nerve fiber pathology

Small fiber pathology is increasingly recognized as a potential contributor to neuropathic pain in different clinical syndromes, however, the underlying mechanisms leading to nociceptor sensitization and degeneration are unclear. With the diversity in clinical pain phenotypes and etiology of small fiber pathology, individual mechanisms are assumed, but are not yet fully understood. The thinly-myelinated Aδ- and unmyelinated C-nerve fibers are mainly affected and clinically require special small fiber test methods to capture functional, morphological, and electrophysiological alterations. Several methods have been established and implemented in clinical practice in the last years. In parallel, experimental and in vitro test systems have been developed allowing important insights into the molecular mechanisms underlying nociceptor sensitization and degeneration as main hallmarks of small fiber pathology. In our narrative review, we focus on these methods and current knowledge, and provide a synopsis of the achievements made so far in this exciting field.

Association of Body Mass Index and Serum Markers of Tissue Damage with Postoperative Pain. The Role of Lactate Dehydrogenase for Postoperative Pain Prediction

Objectives

i) To analyze the association of body mass index (BMI) and some serum tissue damage markers with postoperative pain. ii) To establish a biochemical marker cutoff point able to predict moderate to severe postoperative pain.

Design

Cross-sectional study.

Subjects

Ninety-six adult male patients from Southern Spain (55 ± 13 years old) who underwent an inguinal hernioplasty.

Methods

Postoperative pain (eight hours after surgery) was assessed through a visual analog scale (VAS). Moderate to severe pain was defined as a VAS > 50 mm. BMI was calculated and medication, alcohol consumption, and smoking habit registered. Eight hours after surgery, some serum markers such as fibrinogen, lactate dehydrogenase (LDH), C-reactive protein, cortisol, creatine kinase, glutamic-pyruvic, glutamic oxaloacetic, and gamma-glutamyltransferase transaminases were determined by standard procedures.

Results

After adjusting for potential confounders, BMI was not associated with postoperative pain (P > 0.05). Serum fibrinogen was associated with greater postoperative pain (β = 0.333, P < 0.05). Serum LDH concentration was strongly associated with greater postoperative pain (β = 0.606, P < 0.001). Alcohol consumption was associated with higher postoperative pain (β = 0.212, P < 0.05). No associations were observed regarding age, tobacco consumption, and the rest of serum markers studied. Serum LDH concentration was able to discriminate between presence/absence of moderate to severe postoperative pain (receiver operating characteristic area under the curve = 0.655, P ≤ 0.01). An LDH concentration >204 IU/L was associated with a three-times increased odds ratio of moderate to severe postoperative pain.

Conclusions

Contrary to expectations, greater BMI was not associated with higher postoperative pain. Notwithstanding, the assessment of serum LDH might provide useful information to predict moderate to severe postoperative pain.

Neuropathic Pain: Delving into the Oxidative Origin and the Possible Implication of Transient Receptor Potential Channels

Currently, neuropathic pain is an underestimated socioeconomic health problem affecting millions of people worldwide, which incidence may increase in the next years due to chronification of several diseases, such as cancer and diabetes. Growing evidence links neuropathic pain present in several disorders [i.e., spinal cord injury (SCI), cancer, diabetes and alcoholism] to central sensitization, as a global result of mitochondrial dysfunction induced by oxidative and nitrosative stress. Additionally, inflammatory signals and the overload in intracellular calcium ion could be also implicated in this complex network that has not yet been elucidated. Recently, calcium channels namely transient receptor potential (TRP) superfamily, including members of the subfamilies A (TRAP1), M (TRPM2 and 7), and V (TRPV1 and 4), have demonstrated to play a role in the nociception mediated by sensory neurons. Therefore, as neuropathic pain could be a consequence of the imbalance between reactive oxygen species and endogen antioxidants, antioxidant supplementation may be a treatment option. This kind of therapy would exert its beneficial action through antioxidant and immunoregulatory functions, optimizing mitochondrial function and even increasing the biogenesis of this vital organelle; on balance, antioxidant supplementation would improve the patient's quality of life. This review seeks to deepen on current knowledge about neuropathic pain, summarizing clinical conditions and probable causes, the relationship existing between oxidative stress, mitochondrial dysfunction and TRP channels activation, and scientific evidence related to antioxidant supplementation.

Ethanol-induced damage to the developing spinal cord: The involvement of CCR2 signaling

Ethanol exposure during development causes fetal alcohol spectrum disorders (FASD). A large body of evidence shows that ethanol produces multiple abnormalities in the developing central nervous system (CNS), such as smaller brain size, reduced volume of cerebral white matter, permanent loss of neurons, and alterations in synaptogenesis and myelinogenesis. The effects of ethanol on the developing spinal cord, however, receive little attention and remain unclear. We used a third trimester equivalent mouse model to investigate the effect of ethanol on the developing spinal cord. Ethanol caused apoptosis and neurodegeneration in the dorsal horn neurons of mice of early postnatal days, which was accompanied by glial activation, macrophage infiltration, and increased expression of CCR2, a receptor for monocyte chemoattractant protein 1 (MCP-1). Ethanol-induced neuronal death during development resulted in permanent loss of spinal cord neurons in adult mice. Ethanol stimulated endoplasmic reticulum (ER) stress and oxidative stress, and activated glycogen synthase kinase 3β (GSK3β) and c-Jun N-terminal kinase (JNK) pathways. Knocking out MCP-1 or CCR2 made mice resistant to ethanol-induced apoptosis, ER stress, glial activation, and activation of GSK3β and JNK. CCR2 knock out offered much better protection against ethanol-induced damage to the spinal cord. Thus, developmental ethanol exposure caused permanent loss of spinal cord neurons and CCR2 signaling played an important role in ethanol neurotoxicity.

The association between frequent alcohol drinking and opioid consumption after abdominal surgery: A retrospective analysis

Aims

It is perceived that patients with a history of frequent alcohol consumption require more opioids for postoperative pain control and experience less postoperative nausea and vomiting than patients without such a history. However, there is scarce evidence supporting this notion. The aim of this study was to assess association between frequent alcohol consumption and opioid requirement for postoperative pain control and occurrence of postoperative nausea and vomiting.

Methods

The medical records for 4143 patients using intravenous patient-control analgesia with opioids after abdominal surgery between January 2010 and September 2013 were obtained, and associations were sought between the cumulative opioid consumption (in intravenous morphine equivalence) per body weight (mg/kg) in the first 2 days after abdominal operation and several demographic and clinical variables by multiple regression analysis. The association between the occurrence of postoperative nausea and vomiting and several demographic and clinical variables was also sought by multiple logistic regression analysis.

Results

Frequent alcohol drinking, among other previously reported factors, was associated with increased opioid consumption for postoperative pain control (p < 0.001). The estimate effect of frequent alcohol drinking was 0.117 mg/kg. Frequent alcohol drinking was also associated with decreased risks of postoperative nausea (odds ratio = 0.59, p = 0.003) and vomiting (odds ratio = 0.49, p = 0.026).

Conclusions

Frequent alcohol drinking was associated with increased opioid consumption for postoperative pain control and decreased risks of postoperative nausea and vomiting after abdominal surgery.

An integrated perspective on diabetic, alcoholic, and drug-induced neuropathy, etiology, and treatment in the US

Neuropathic pain (NeuP) is a syndrome that results from damaged nerves and/or aberrant regeneration. Common etiologies of neuropathy include chronic illnesses and medication use. Chronic disorders, such as diabetes and alcoholism, can cause neuronal injury and consequently NeuP. Certain medications with antineoplastic effects also carry an exquisitely high risk for neuropathy. These culprits are a few of many that are fueling the NeuP epidemic, which currently affects 7%-10% of the population. It has been estimated that approximately 10% and 7% of US adults carry a diagnosis of diabetes and alcohol disorder, respectively. Despite its pervasiveness, many physicians are unfamiliar with adequate treatment of NeuP, partly due to the few reviews that are available that have integrated basic science and clinical practice. In light of the recent Centers for Disease Control and Prevention guidelines that advise against the routine use of μ-opioid receptor-selective opioids for chronic pain management, such a review is timely. Here, we provide a succinct overview of the etiology and treatment options of diabetic and alcohol- and drug-induced neuropathy, three different and prevalent neuropathies fusing the combined clinical and preclinical pharmacological expertise in NeuP of the authors. We discuss the anatomy of pain and pain transmission, with special attention to key ion channels, receptors, and neurotransmitters. An understanding of pain neurophysiology will lead to a better understanding of the rationale for the effectiveness of current treatment options, and may lead to better diagnostic tools to help distinguish types of neuropathy. We close with a discussion of ongoing research efforts to develop additional treatments for NeuP.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Alcoholic neuropathy: possible mechanisms and future treatment possibilities

Chronic alcohol consumption produces painful peripheral neuropathy for which there is no reliable successful therapy, mainly due to lack of understanding of its pathobiology. Alcoholic neuropathy involves coasting caused by damage to nerves that results from long term excessive drinking of alcohol and is characterized by spontaneous burning pain, hyperalgesia and allodynia. The mechanism behind alcoholic neuropathy is not well understood, but several explanations have been proposed. These include activation of spinal cord microglia after chronic alcohol consumption, oxidative stress leading to free radical damage to nerves, activation of mGlu5 receptors in the spinal cord and activation of the sympathoadrenal and hypothalamo-pituitary-adrenal (HPA) axis. Nutritional deficiency (especially thiamine deficiency) and/or the direct toxic effect of alcohol or both have also been implicated in alcohol-induced neuropathic pain. Treatment is directed towards halting further damage to the peripheral nerves and restoring their normal functioning. This can be achieved by alcohol abstinence and a nutritionally balanced diet supplemented by all B vitamins. However, in the setting of ongoing alcohol use, vitamin supplementation alone has not been convincingly shown to be sufficient for improvement in most patients. The present review is focused around the multiple pathways involved in the development of peripheral neuropathy associated with chronic alcohol intake and the different therapeutic agents which may find a place in the therapeutic armamentarium for both prevention and management of alcoholic neuropathy.

Subdiaphragmatic vagotomy promotes nociceptive sensitivity of deep tissue in rats

To verify whether vagal dysfunction is associated with chronic pain, we evaluated the effects of subdiaphragmatic vagotomy (vgx) on the sensitivity toward noxious stimuli in rats. Vgx rats showed sustained hyperalgesia in the gastrocnemius muscle without tissue damage (no increase in vgx-induced plasma creatine phosphokinase or lactose dehydrogenase levels) accompanied by hypersensitivity to colonic distension. We found a dramatic increase in the levels of metabotropic glutamate receptor 5, protein kinase C (PKC) gamma and phosphorylated-PKCgamma within the spinal cord dorsal horn in vgx rats, which suggests that vgx may evoke sensory nerve plasticity. Morphine produced a dose-dependent increase in the withdrawal threshold in both vgx and sham-operated rats, but the effect of a lower dose in vgx rats was weaker than that in sham-operated rats. Muscle hyperalgesia in vgx rats was also attenuated by gabapentin and amitriptyline, but was not affected by diclofenac, dexamethasone or diazepam. These findings indicate that subdiaphragmatic vagal dysfunction caused chronic muscle hyperalgesia accompanied by visceral pain and both gabapentin and amitriptyline were effective for subdiaphragmatic vagotomy-induced pain, which are partially similar to fibromyalgia syndrome. Furthermore, this chronic muscle pain may result from nociceptive neuroplasticity of the spinal cord dorsal horn.

Tocotrienol ameliorates behavioral and biochemical alterations in the rat model of alcoholic neuropathy

Chronic alcohol consumption produces a painful peripheral neuropathy for which there is no reliable successful therapy, which is mainly due to lack of understanding of its pathobiology. Alcoholic neuropathy is characterized by spontaneous burning pain, hyperalgesia (an exaggerated pain in response to painful stimuli) and allodynia (a pain evoked by normally innocuous stimuli). Chronic alcohol intake is known to decrease the nociceptive threshold with increased oxidative-nitrosative stress and release of proinflammatory cytokines coupled with activation of protein kinase C. The aim of the present study is to investigate the effect of both isoforms of vitamin E, alpha-tocopherol (100mg/kg; oral gavage) and tocotrienol (50, 100 and 200mg/kg; oral gavage) against alcohol-induced neuropathic pain in rats. Male Wistar rats, were administered 35% v/v ethanol (10 g/kg; oral gavage) for 10 weeks, and were treated with alpha-tocopherol and tocotrienol for the same duration. Ethanol-treated animals showed a significant decrease in nociceptive threshold as evident from decreased tail flick latency (thermal hyperalgesia) and decreased paw-withdrawal threshold in Randall-Sellito test (mechanical hyperalgesia) and von-Frey hair test (mechanical allodynia) along with the reduction in nerve glutathione and superoxide dismutase levels. TNF-alpha and IL-1beta levels were also significantly increased in both serum and sciatic nerve of ethanol-treated rats. Treatment with alpha-tocopherol and tocotrienol for 10 weeks significantly improved all the above-stated functional and biochemical deficits in a dose-dependent manner with more potent effects observed with tocotrienol. The study demonstrates the effectiveness of tocotrienol in attenuation of alcoholic neuropathy.

Nociceptive and behavioural sensitisation by protein kinase Cepsilon signalling in the CNS

Despite the apparent homology in the protein kinase C (PKC) family, it has become clear that slight structural differences are sufficient to have unique signalling properties for each individual isoform. For PKCepsilon in depth investigation of these aspects revealed unique actions in the CNS and lead to development of specific modulators with clinical perspective. In this review, we describe to which extent PKCepsilon is distinct from other isoforms on the level of tissue expression and protein structure. As this kinase is highly expressed in the brain, we outline three main aspects of PKCepsilon signalling in the CNS. First, its ability to alter the permeability of N-type Ca2+ channels in dorsal root ganglia has been shown to enhance nociception. Secondly, PKCepsilon increases anxiety by diminishing GABA(A)R-induced inhibitory post-synaptic currents in the prefrontal cortex. Another important aspect of the latter inhibition is the reduced sensitivity of GABA(A) receptors to ethanol, a mechanism potentially contributing to abuse. A third signalling cascade improves cognitive functions by facilitating cholinergic signalling in the hippocampus. Collectively, these findings point to a physical and behavioural sensitising role for this kinase.